Pitting and corrosion of hands?

[Etouffee]

New here. Help me understand, I’ve been looking at a lot of vintage watches and read and see a lot of pitted and corroded minute, hour and second hands. How does this happen to water proof/resistant watches? Frankly, some not that old... I’ve been lurking here a while but have not seen a “why”. Is this due to poor watch maintenance? What causes this? Just age?

Seems a sealed unit, this shouldn’t happen.

I’m sure this has been addressed, please forgive a redundancy...

Thanks for your enlightenment. (Be kind, 1st post)